1. Field of the Invention
The present invention relates to a resin molded product, having excellent vibration-damping properties, which is useful as an engine part or attachment.
2. Description of the Prior Art
The substitution of plastic components and parts for metallic components and parts in the engines of motor vehicles, such as automobiles, has been a field of much interest in recent years due to the need for smaller fuel consumption in motor vehicles.
Nylon resin materials are superior to metallic materials with respect to providing light-weight products. In addition, nylon resin materials have various excellent characteristics with respect to vibration-damping properties, rigidity, heat resistance, oil resistance, and the like.
For this reason, nylon resins have recently been used in the production of engine parts and attachments, such as the cylinder head cover, gearcase, etc., in order to reduce the weight, as well as the noise, of the motor vehicle.
In general, resin materials are superior to metallic materials with respect to the vibration-damping performance of the components obtained therefrom. However, with regard to the noise reduction effect, the use of resinous components results in an increase in air-transmitted sounds due to the small specific gravities thereof, although solid-transmitted sounds (i.e., sounds caused by the vibration of the components themselves) are reduced. Therefore, in order to effectively reduce noise when replacing metallic components with resinous components, the resulting increase in air-transmitted noise must be more than counterbalanced by the reduction in solid-transmitted noise. In other words, resinous components are required to have an extremely high vibration-damping performance.
Resin materials generally possess high viscoelastic properties, with their vibration-damping effect being maximum around temperatures at which the dissipation factors (tan .delta.) of the mechanical dispersion, mainly due to the glass transition of the polymer materials, have maximal values. Such temperatures are usually 10.degree. to 30.degree. C. higher than the glass transition temperatures (Tg) measured by differential scanning calorimetry (DSC).
For example, straight-chain aliphatic nylons such as nylon 6, nylon 66, nylon 12, nylon 11, and nylon 610 have a glass transition temperature (Tg) of from about 40.degree. to about 60.degree. C., and their vibration-damping performance is maximum at a Tg of from about 60.degree. to 80.degree. C.
Aromatic nylons such as crystalline m-xylylenediamine-based resin (MXD nylon), which has aromatic rings in its molecular chains, and a copolymer of one or more diamines and one or more dicarboxylic acids (e.g., terephthalic acid and isophthalic acid), which is a non-crystalline nylon, have a Tg of 120.degree. C. or more, with their vibration-damping effect being maximum at temperatures as high as 130.degree. C. or more.
In addition to the requirement that automobiles be light weight, there is a growing demand that automobiles be more comfortable. One factor that contributes to an improvement in comfort is the reduction of vibration noise. Such vibration noise can be attributed to the vibration of engine parts as well as various resinous molded products (examples of which are set forth below) which are attached to the engine and engine parts by bolts, nuts, screws, etc., with the vibrations of these products being induced by the vibration of the engine.
Since the resinous molded products are located around the engine and thus are generally heated to a temperature in the range of 80.degree. to 120.degree. C., they must have excellent vibration-damping properties, especially in that temperature range.